1. Field of the Invention
This invention is directed to compositions and a method. More specifically, this invention concerns itself with ultraviolet light sensitive polymer compositions having enhanced resistance to photodegradation, and methods for enhancing the resistance of ultraviolet light sensitive polymers to photodegradation.
2. Description of the Prior Art
Polymers have in the past and continue to provide an attractive substitute for other more traditional types of structural materials (e.g. wood and metals) because of relatively inexpensive materials and fabrication costs. As polymers continue to find new applications in, for example, the fabrication of automotive parts and building materials, they must also become more durable and capable of withstanding prolonged exposure to a variety of degradative forces. Degradation of polymers can be caused by exposure to light, heat and/or air. Such degradation is usually manifest by either a partial or total loss of structural integrity, changes in light transmission properties, changes in color, loss or reduction in flexibility and/or resiliency, or any combination of the above phenomenon. Those attempting to avoid polymer degradation have generally selected from among three possible approaches: (a) elimination or reduction of the degradative forces; (b) isolation of the sensitive polymer material from the degradative forces; or (c) modification of the polymer composition to enhance its resistance to degradative forces. The latter approach is generally preferable since it does not require elaborate engineering or structural changes in the polymer product environment.
There are a variety of additives which have been disclosed as suitable for enhancing the resistance of polymers to one or more of the degradative forces discussed hereinabove. These additives (hereinafter referred to as "stabilizers") can be physically combined with or engrafted upon the environmentally sensitive polymer, thereby prolonging its useful life in the hostile degradative environment. In those instances where the stabilizer is merely physically dispersed throughout the polymer, it is generally free to migrate within the composition (unless, of course, its relative molecular size inhibits such diffusion). As will be appreciated, this problem of stabilizer diffusion can be further aggravated when the polymer composition is in continuous or periodically contacted with fluids. Such fluid contact can result in the leeching out or removal of some of the stabilizer from the polymer by the solvent action of the fluid and/or the anisotropic distribution of the stabilizer within the polymer composition. The removal and/or redistribution of stabilizer within the polymer can leave the unstabilized region of the polymer vulnerable to attack by degradative forces. Thus, it will be appreciated that stabilizers which are inhibited from migration and/or diffusion within the polymer are generally preferred.
Stabilizers are available which can enhance the polymers resistance to more than one of the degradative forces and conversely, a stabilizer which is effective for prevention of, for example, oxidative degradation may have little if any effect upon the polymers resistance to other degradative agents. Thus, it is not uncommon for polymers to contain a variety of stabilizer materials, each being present for the prevention of a particular degradative reaction.
One of the more difficult to control of the degradative forces is irradiation of the polymer by ultraviolet light. The impact of such irradiation will of course vary depending upon the intensity and duration of exposure and thus may manifest itself only after a prolonged interval. The irradiation of polymers with ultraviolet light can often times cause cross-linking of these materials thereby reducing its resiliency and/or impact resistance. Changes in color and opacity are also often affected by prolonged exposure of the polymer to ultraviolet light. While many materials are known, and commercially available, as stabilizers against ultraviolet light degradation, the degree of protection afforded by such agents is often concentration dependent. The following list of references are illustrative of the types of ultraviolet absorbers commonly used in conjunction with polymeric materials sensitive to photodegradation: U.S. Pat. Nos. 3,362,929; 3,362,930; 3,829,292; 3,901,849; 3,910,918; 3,939,164; 4,028,334; published patent application Nos. B402,162 and B571,638; U.K. Pat. No. 999,806; and an article appearing in J. AM. CHEM. SOC., Vol. 60: 1458 et seq. (1938). In addition, commonly assigned U.S. patent application Ser. Nos. 697,345 (now U.S. Pat. No. 4,073,770) and 697,387 (now U.S. Pat. No. 4,069,195) (both filed on June 18, 1976) disclose compounds which are highly effective as ultraviolet light stabilizers for polymeric materials. The compounds disclosed in the above referenced pending patent applications are substituted decahydroquinolines and are highly effective in enhancing the resistance of polyolefins to photodegradation by ultraviolet light.
As will be readily appreciated, the addition of substantial amounts (in excess of 10 weight percent) of stabilizer materials to a polymer can often modify its intrinsic physical, chemical and/or electrical properties or the intrinsic properties of articles prepared therefrom. Moreover, the problems related to stabilizer diffusion complicate the effectiveness of the stabilization technology, especially where fluid contact of the polymer composition is contemplated. Thus, there is a continuing need for highly efficient ultraviolet light stabilizers which are both effective at low concentrations and resistant to diffusion and/or leeching by fluids which may come in contact with the polymer composition.